The present invention relates to a biological conversion of xylulose to ethanol by fermenting xylulose with a yeast under fermentative conditions. Fermentative conditions are generally recognized to be the conditions largely anaerobic, but sometimes with limited oxygen supply, and which are suitable for producing ethanol.
With petroleum resources dwindling throughout the world, efforts are being made to utilize alternative sources of energy. Recently, mixtures of gasoline and alcohol have been touted as but one means of alternate energy savings. In order to provide sufficient gasohol products, an economic means for producing the necessary alcohol is needed. Heretofore, production of such alcohol has been chiefly carried out by the conversion of six-carbon sugars derived mainly from grain products and cane sugar juice or molasses. With the increase in value of such six-carbon sugar materials, there is a need for alternative economic feedstocks for the production of alcohol. Much of the available hemicellulosic feedstock which has been regarded as waste products, such as bagasse, remain untapped due to the lack of a viable economic means to convert such hemicellulosic waste materials to useful energy forms such as ethanol.
Numerous methods are known for producing small amounts of ethanol from six-carbon sugars under submerged cultural conditions using mycelial fungi as illustrated for example in U.S. Pat. Nos. 3,936,353; 3,591,454; 2,556,084; 1,572,539; 1,472,344, and 1,266,657. These methods however, do not provide a means for the conversion of five-carbon monosaccharides such as xylose and xylulose to ethanol.
Many yeasts are capable of fermenting hexoses to ethanol anaerobically with high yields. However, no yeasts have been reported to ferment pentoses to ethanol, even though many yeasts are capable of metabolizing pentoses aerobically. Therefore, the pentoses which are derived primarily from hemicellulosic materials have been reported to be nonfermentable sugars. Several bacteria (e.g., Clostridia) and mycelial fungi (e.g., Mucor spp., Rhizopus spp., and Fusaria) are known to metabolize as well as ferment pentoses to ethanol. For ethanol production from biomass, yeasts are preferred over bacteria and mycelial fungi. The yeast process for fermenting glucose to ethanol is a relatively simple well-studied process. Clostridia fermentation of pentoses are not satisfactory due to the low yield and generating organic acid (e.g., acetic acid) as fermentation products along with ethanol. See for example U.S. Pat. No. 1,857,429.
The biological conversion of five-carbon sugars derived from hemicellulose to ethanol is important to fully utilize biomass to produce liquid fuels, especially in view of the fact that vast quantities of hemicellulosic materials are readily available, but under-utilized due to the lack of the ability of proper organisms to convert pentoses to ethanol efficienty. The research into possible large-scale utilization of hemicellulose derived sugars as substrates to convert it to ethanol has thus received little attention. One major reason for this inadequacy is that the fundamental knowledge concerning the metabolism of pentoses by microorganisms and converting it into ethanol is insufficient.
The utilization and conversion of pentoses into ethanol requires the involvement of enzymes which are not required for D-glucose fermentation.
It is a primary object of the present invention to provide a new technique for the production of ethanol from xylose and xylulose materials.
It is a further object of the present invention to provide a means for production of ethanol from xylose and xylulose which is compatible with conversion of six-carbon sugars.
These and other objects will be more apparent from the discussion below.